Created it, 06/10/19
Update it, 06/10/25
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7. - FOURTH EXPERIMENT : “ROCKER D IN A CIRCUIT USED FOR THE INDICATION OF A FIRST EVENT”
With the experiments undertaken until now, you had the occasion to examine the operation and the characteristics of various types of integrated circuits. They were the combinative AND, NAND, NOR logical circuits, OR, EXCLUSIVE-OR, of the reversers and the rockers.
These experiments were necessary in order to know the basic components used in the digital circuits. That perhaps seemed to you monotonous, just as the training, however necessary to the resolution of the problems, the four fundamental operations at the school.
When you discover the possibilities of concrete applications, numerical electronics will become captivating then.
With your knowledge on the basic logical integrated circuits, you are able to carry out practical, simple circuits at the beginning, more complexes then.
With a rocker D, you will carry out a circuit which will detect the push-button inserted the first.
A circuit of this type finds an application for example in the television games, to indicate which of the two players pressed the first on the button.
In the industrial field, it is a question of determining which event occurs before another.
7. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and remove matrix and group of connectors the whole of the connections and the integrated circuit assembled during the preceding experiment.
b) Take an integrated circuit MM 74C74 (double rocker D) and the MM 74C08 (quadruple AND) and insert them on the matrix in the position indicated to the figure 20-a.
c) Carry out the connections also indicated to the figure 20-a.
d) Put switch SW0 on position 0.
By observing the electric diagram of the circuit carried out (figure 20-b) you notice this :
the entries DATED
from the two rockers are cabled with the positive tension, thus a positive
transition on the entry from clock CLOCK
transfers a logical level 1 on the exit Q.
entries CLEAR
are connected together to switch SW0. It is
enough to put it on position 0 to have a
logical level 0 on the two exits Q1
and Q2.
entries PRESET
are cabled with the positive tension and thus do not have any influence on the
operation of the rockers.
Entries CLOCK
are connected through two circuits AND to
the contacts P0
and P1.
the exits Q1 and
Q2 order respectively the LED
L0 and L1.
7. 2. - CONTROL OF OPERATION
a) Connect the food. You note that the two LED L0 and L1 are extinct. Indeed, SW0 is in position 0 and two entries CLEAR are active ; the exits Q1 and Q2 are thus on the level L.
b) Put SW0 on position 1 : entries CLEAR become inactive.
c) Actuate the P0 button : the LED L0 ignites.
d) Actuate the P1 button : the situation remains unchanged, L0 lit, L1 extinct.
e) Remettez SW0 on position 0 then bring back SW0 on position 1. You find the initial state, i.e. the two extinct LED.
f) Actuate the P1 button : the LED L1 ignites.
g) actuate the P0 button : the situation does not change.
h) again give SW0 on position 0 then on position 1.
i) Repeat several times this same experiment. Each time, you note that it is the first inserted button which is taken into account by the circuit. The second button that you insert does not produce any effect on the two exits Q1 and Q2.
Let us analyze the operation of the
circuit. By putting SW0 on position 0,
you obtain an initial condition under which Q1
and Q2 are on the level L,
1
and 2
is on the level H.
In this situation, the two circuits AND are validated, because one of their two entries being on a level H, they can transfer at exit the transition from L to H provided by the buttons P0 and P1.
The first actuated button generates an impulse which is applied to entry CLOCK through the circuit AND correspondent.
The exit of the corresponding rocker passes on level 1 since the two entries DATED are to 1.
The exit
of this rocker passes to 0.
If you support on P0,
Q1 passes on the level H
and 1
on the level L.
Thus, the circuit AND on which arrives the impulse of the second P1 button is blocked. Entry CLOCK of the second rocker is not activated.
It is the same if you support on P1
and then on P0. It is exit 2
at the level 0 which blocks the circuit AND
located on entry CLOCK 1 of the first
rocker.
Switch SW0 each time enables you to find yourselves under the initial conditions.
8. - FIFTH EXPERIMENT : “REALIZATION OF A CIRCUIT A COMBINATIONS”
The following circuit enables you to know if four switches were actuated one after the other in a definite order. In the event of error, the circuit is blocked.
This type of circuit is used for the opening of the doors equipped with a combination lock. While pressing on the buttons in the exact order, the lock opens.
8. 1. - REALIZATION OF THE CIRCUIT
a) Disconnect the food and dismount the circuit carried out completely previously.
b) Take two integrated circuits MM 74C74 (double rocker D), a circuit MM 74C00 (quadruple NAND) and a circuit MM 74C08 (quadruple AND) and introduce these circuits on the matrix as indicated in the figure 21-a.
c) Carry out the connections indicated to the figure 21-a by omitting for the moment those traced in dotted line.
d) Put the switches on position 0.
The circuit carried out is schematized by the figure 21-b. It can appear complex to you but makes some, it is rather simple. The two circuits MM 74C74 are differentiated ; one is noted MM 74C74-1, the other MM 74C74-2.
Entries PRESET of the four rockers are cabled with the positive tension, therefore inactive.
The entries DATED are cabled in the same way with the positive tension ; entries CLOCK are connected to the switches in the following way :
first rocker of the circuit MM 74C74-1 with SW1
first rocker of the circuit MM 74C74-2 with SW3
second rocker of the circuit MM 74C74-1 with SW0
second rocker of the circuit MM 74C74-2 with SW2
By putting a switch on position 1,
a positive transition is applied to entry CLOCK
from the rocker and the entry DATED is
transferred at exit Q. Q
passes on the level H,
on the level L.
A network made up of three circuits NAND
and three circuits AND is connected to three
switches SW3, SW0, SW2 and at the three
exits
of the first three rockers. The output signal of the network orders entries CLEAR
of the four rockers.
During handling, you will study the function of this network.
The output signal of the circuit is the exit Q of the fourth rocker and order the LED L7.
8. 2. - CONTROL OF OPERATION
a) Connect the food : the LED L7 is extinct. If not, support on P0 to extinguish L7. You thus obtain the desired initial conditions.
b) Put the switches on position 1 in the following order : SW0, SW1, SW2, SW3 : the situation is unchanged, the LED L7 remains extinct.
c) Give the switches on position 0, then reposition them on level 1 in the following order : SW1, SW3, SW0, SW2. The LED L7 ignites.
d) Give the switches on position 0 : L7 remains lit.
e) Support on P0 : L7 dies out, you are under the desired initial conditions.
f) Carry out several tests by successively positioning the four switches out of 1 in an unspecified order. Only sequence SW1, SW3, SW0, SW2 makes it possible to light L7.
In conclusion, this logical circuit recognizes only one combination among the 24 possible combinations with four switches.
g) Disconnect the food.
IMPORTANT NOTE :
If the circuit does not function as envisaged, checks the connections carried out attentively and ensures you that the drivers are well inserted in the various contacts of the matrix and the group of connectors. Also check the insertion of the integrated circuits in the matrix, on the level of their orientation and their pins.
In the event of failure, you can suspect a defective integrated circuit. In this case, check the individual operation of each integrated circuit using the various assemblies of study carried out before for each type of integrated circuit.
To include/understand the operation of the circuit carried out, carry out the connections indicated into dotted on the figure 21-a. You can thus visualize the tensions in certain points of the circuit thanks to the LED L0, L1, L2, L4.
a) Connect the food, put the four switches on position 0 and support on P0 ; you observe this :
L0
is lit, therefore the exit
of the first rocker is on the level H
L1
is lit, the exit
of the second rocker is on the level H
L2
is lit, the exit
of the third rocker is on the level H
L4 is lit, entry CLEAR of the four rockers is on the level H.
Under these initial conditions, the four rockers are with state 0, ready to receive the correct sequence of the four switches.
b) Now put SW1
on position 1. The
LED L0 dies out, the first rocker transferred a level H
at exit Q and
is thus on the level L.
c) Put the three other
switches on position 1 in order SW3,
SW0, SW2. The LED L1, L2 die out
and finally L7 ignites. While positioning SW3
and SW0, the second and third rocker passed
on the level H and the exits
passed on the level L. the action on SW2
transfers a level H at exit Q,
L7 ignites.
The exact sequence is thus confirmed by the successive extinction of L0, L1, L2 and the lighting of L7 and L4.
d) Give the four switches on position 0 and support on P0.
You note that L4 dies out, a level L applied to entries CLEAR gives to zero four rockers (LED L0, L1, L2 lit, L7 extinct).
Here now what occurs when the order is not respected.
e) Put SW1 on position 1 : the LED L0 dies out.
f) Now put SW0 on position 1 instead of SW3.
The LED L4 dies out, the four rockers are given to zero by entries CLEAR. The circuit is blocked and by actuating other switches, SW3 and SW2, there is no possibility of releasing of the circuit. This result is obtained with the network of NAND and of AND which presents a level L at exit and force the rockers at state 0.
The examination of the circuit being finished, disconnect the food and remove the connections indicated into dotted to the figure 21-a or 21-b.
In conclusion, the operation of the circuit can be summarized as follows :
by putting the four switches on position 0
and while supporting on P0, the circuit is
predisposed for operation.
by actuating the four switches in the given sequence
(SW1, SW3, SW0, SW2), the circuit allows the
lighting of the LED L7.
any other sequence is not recognized by the circuit
and the pilot LED L7 remains extinct.
In addition, by changing designations of the four switches, you can modify the combination. It is possible also to increase the number of switches by adding a rocker for a new switch. It is necessary to add circuits NAND and AND and the total circuit becomes more complex.
This circuit illustrates in the most complete way the concept of circuit with memory. A circuit only made up of logical doors cannot recognize a sequence, on the other hand, a circuit made up of rockers is able to remember the last events.
The examined case is particularly clear : each rocker memorizes an event, made up by closing of the corresponding switch.
Note that the connections illustrated in dotted line are used to visualize the tensions in certain points of the circuit and do not exploit any part the operation of the circuit.
You will have always faculty to establish this type of connections to contribute you to the comprehension of a circuit.
In the next practice, you will approach the examination of the monostable one.
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